Layered arrangement and component containing the latter

ABSTRACT

The invention concerns a layered arrangement comprising at least one layer based on a high-temperature superconductive material with at least one unit cell having a CuO 2  plane, the layer being connected to a non-supeconductive layer. A modified interface layer is provided between the two layers. Alternatively, at least one of the contacting layers can be modified in the interface region. Modification can be brought about by doping with metallic ions or implantation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT/DE97/01812 filed Aug. 22,1997 and based upon German national application 196 34 118.3 of Aug. 23,1996 under the International Convention.

FIELD OF THE INVENTION

The invention relates to a layered arrangement in solid state systems.The invention also relates to a cryogenic component.

BACKGROUND OF THE INVENTION

The use of high temperature superconductors for incorporation incryo-electronic components required the development of integratedmultilayer circuits. Examples of superconductor integrated multilayercircuits are SNS-Josephson contacts or flux transformers. In addition,for producing a flux transformer for example, throughgoing contacts orstrip conductor crossovers are frequently used.

Josephson contacts, through contacts and conductive strip crossoversare, in addition, the main components for a series of further multilayercomponents and are significant for cryo-electronics in general. Thecharacteristics of such components in layered technology, for example,Josephson contacts, are a function of the barriers between thesuperconductive contacts through the barrier material and are notdetermined by the interfacial resistance of the boundary layer regionsbetween the mutually-contacting layers.

For the desired suppression of interfacial resistances in such contacts,clean ex-situ fabricated boundary layers of high temperaturesuperconductor materials like YBa₂Cu₃O_(7-x) or PrBa₂Cu₃O_(7-x) areprepared, for example, by chemical etching in nonaqueous Br-ethanolsolutions {M. I. Faley, U. Poppe, H. Soltner, C. L. Jia, M. Siegel, andK. Urban, “Josephson Junctions, Interconnects and Crossovers onChemically Etched Edges of YBa₂Cu₃O₇””.—Appl. Phys. Lett., v. 63, No.15, 2138-2140 (1993)}.

The chemical etching produces not only structurally undamaged boundarysurfaces but also small edge angles of about 3°. The use of theseshallow edges after structuring of the high temperature superconductorthin film is significant for two reasons. On the one hand, this edgestructure supports current transport in the c-direction with stronglyanisotropic high temperature conductors and provides a solution to theproblems of current transport. On the other hand, such edge structuresallow a relatively homogeneous covering of the base electrode by aninsulating or nonsuperconductive layer.

The homogeneous covering or coating of the base electrode by a thinbarrier layer is also of significance in the production of Josephsoncontacts. Ideally through-contacts (vias) should have the smallestpossible interfacial resistance while, by comparison, good insulatingcharacteristics are required for them in the production of conductivestrip crossovers. Up to now, for insulation, cubic materials like forexample SrTiO₃ or CeO₂ were often used. These materials have thedrawback that the superconductive base electrode is frequently deficientin oxygen and may not have sufficient superconductive characteristics.Because of this drawback, the advantageous nonaqueous chemical etchingin Br-ethanol for conductive strip crossovers or Josephson contactscannot be used.

OBJECT OF THE INVENTION

It is therefore the object of the invention to provide a layeredarrangement in which the quality of the boundary surface regions betweenthe layers can be adjustable in a targeted manner in dependence upon thedesired boundary conditions.

SUMMARY OF THE INVENTION

This object is achieved with a layered arrangement containing at leastone layer on the basis of a high temperature superconductive materialwith at least one C_(u)O₂ plane forming unit cells. The layer is bondedto a nonsuperconducting layer. A modified interfacial layer is providedbetween the two layers. The object is further achieved with a cryogeniccomponent having such a multilayer system.

According to the invention the high temperature superconductive materiallayer is bonded with a nonsuperconducting layer via a modifiedinterfacial region at least on one of the mutually contacting layers.The surface region or the interfacial layer can be doped with ions,especially with metal ions for the modification. The surface region orthe interfacial layer can be implanted with ions, especially with metalions for the modification. The nonsuperconductive layer on its surfaceturned away the superconductive layer can also be modified in itssurface region. Advantageously the nonsuperconductive layer on itssurface turned away the superconductive layer is bonded to a furtherhigh temperature superconductive layer. PrBa₂Cu₃O_(7-x) is a preferredmaterial of the nonsuperconductive layer. A multilayer system with aplurality of layered arrangements as described above can form a throughcontact arrangement or a conductive strip crossover.

It has been found that in this way it is possible to produce anepitaxial multilayer system with high temperature superconductors forintegrated cryo-electronic circuits which is especially suitable forproducing Josephson contacts, through contacts or conductive stripcrossovers. By contrast with earlier known comparable structures, it ispossible to obtain in an advantageous manner improved superconductivecharacteristics within the overall heterostructure.

The layered arrangement according to the invention or the componentaccording to the invention has a homogeneous epitaxial growth and acomplete oxidation of the layer sequence, especially the multilayerstructure. These requirements are fulfilled by the use of oxidicmaterials which are technically, chemically and structurally compatiblewith one another. It has been found that the oxidic high temperaturesuperconductors basically have a charge carrier density which iscomparable with that of highly doped semiconductors. The spatialvariation in the charge carrier density, for example at boundary layers,can be, by comparison to customary metals, extend relatively largedistances up to about 100 nm into the layer. As a result, the transportcharacteristics of thin oxidic materials can be strongly influenced bythe behavior in the region of the interface between the respectivelayers.

With the aid of a modification of one or more of these boundary layersand thus of the adjustability of the value of the interfacial resistanceover a wide range of values, it is possible to fabricate integratedmultilayer systems based upon high temperature superconductors whichfulfill the individual function-determining requirements of suchcryo-electronic components.

A targeted variation or adjustability of the interfacial resistance canbe produced by a targeted manipulation of the boundary layer in thismanner. Thus different materials which are technologically, chemicallyand structurally compatible with one another can be used.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described in greater detail in connection with Figuresand an example explained below. In the drawing:

FIG. 1 is a high resolution transmission electron microscopic image ofan intermediate layer at the boundary of a layered arrangement accordingto the invention and which is traceable to ion beam etching of thePrBa₂Cu₃O_(7-x)-layer;

FIG. 2 plots specific resistance along the c-axis for a 0.5 μm thickPrBa₂Cu₃O_(7-x)-insulation layer arranged between two Yba₂Cu₃O_(7-x)-electrodes; and

FIG. 3 is a side view of a Josephson contact according to the inventionwith modified boundary layer regions.

SPECIFIC DESCRIPTION AND EXAMPLE

FIG. 1 shows a TEM-picture of a layer sequence according to theinvention. Visible therein is a modified intermediate layer at theinterface between superconductive and nonsuperconductive layers of thelayer arrangement according to the invention. This intermediate layer isobtained by an ion beam etching of the nonsuperconductive layerPrBa₂Cu₃O_(7-x) before the formation of the superconductive layerapplied to this layer.

In this manner a clean, undistorted boundary surface is obtained beforemodification of the nonsuperconductive-free layer surface by the use ofa chemical etching in Br-ethanol in combination with photolithography bymeans of hard UV radiation in a known manner.

Structurally modified or damaged boundary surfaces are then produced bythe action of the surface of oxidic material with an ion beam,especially with an energy of 600 eV. By varying the irradiation durationfor the ion energy, desired boundary conditions of the modification anddifferent degrees of damage at the boundary surfaces can be generated.In the case of epitaxial heterostructures of the high temperaturesuperconductor YBa₂Cu₃O_(7-x) thin films, for example by the combinationof chemical etching with ion beam etching, interfacial resistancesR_(n)A in the range of R_(n)A≈100 Ωcm² to less than 1×10⁻¹⁰ Ωcm² can beobtained. The value of A is the magnitude of the contact area.

Different and especially high values for the interfacial resistance canbe generated alternatively or cumulatively by different modifications ormanipulations of the boundary surfaces of the materials in a thin layersurface region, especially up to 10 nm, at the interface, especially bydoping with different metal ions or by partial substitution of theoxygen, for example with F, Br, Cl.

An important recognition has been that with such interfacialmodifications, a thin epitactic intermediate layer is obtained at theinterface which can form an elevated interfacial resistance. Themodified interface then influences the charge carrier density inelectrode materials, especially the transport characteristic ofYBa₂Cu₃O_(7-x) in the region (up to 100 nm) of the boundary surfaces ofthe layer arrangement according to the invention here given as anexample.

In FIG. 2, the specific resistance along the c-axis for 0.4 μm thickPrBa₂Cu₃O_(7-x) “insulation” layers between two YBa₂Cu₃O_(7-x)electrodes is given. The measured values are shown for the purelychemically-etched boundary surface by the curve “a” and at “b” for theion beam etching dislocated boundary surface. The curves “c” and “d”indicate the specific resistance of PrBa₂CU_(2.85)Ga_(0.15)O_(7-x) andPrBa₂Cu₃O_(7-x) filaments ab copperoxide planes of these materials. Theinset in the Figure mentioned shows the temperature dependency of theJosephson contact resistance R_(n)(T) for contacts with barrierthicknesses of 10 nm, 20 nm, 30 nm and 100 nm (shown from bottom totop).

The modification of the interfacial resistance at the boundary surfacesbetween the YBa₂Cu₃O_(7-x) layer and the PrBa₂Cu₃O_(7-x) layer by argonion etching enables the increase in the insulation resistance of thePrBa₂Cu₃O_(7-x) insulation by up to four orders of magnitude (see FIG.2). In this manner multilayer flux transformers of high quality can beproduced.

A PrBa₂Cu₃O_(7-x) layer between two YBa₂Cu₃O_(7-x) electrodes withundamaged boundary surfaces shown, in a comparison to a massive probe, avery small specific resistance (inset in FIG. 2). This is advantageousfor Josephson contacts because one can use in this manner relativelylarge barrier thicknesses (>10 nm) which is larger than typical surfaceroughnesses and as a result can produce a homogeneous current densitydistribution in Josephson contacts.

In FIG. 3 a Josephson contact according to the invention is shown inside view with a modified boundary layer region. A first superconductorelectrode of YBa₂Cu₃O_(7-x) is then formed on a substrate of LaAlO₃. Afree interface I is then cleaned and modified to form a high surfaceresistance. On this is applied thereafter a nonsuperconductive layer ofPrBa₂Cu₃O₇ whose upper surface II is also modified on the same ground.

Chemical etching is then carried out at an angle α of 3° to etch a slopetherein and the free surface II of nonsuperconductor layer is coveredwith a thin (10-100 nm) layer of nonsuperconductive PrBa₂Cu₃O₇. Finallya further superconductive layer of YBa₂Cu₃O₇ is formed, preferably witha layer thickness in the range of 100 to 300 nm. On the slope in theregion between both superconductor layers, a Josephson contact is thenformed. The modification at the interfacial regions I and II provides acomponent with improved electronic characteristics since it has a betterelectrical insulation in the region of the components, where a currentflow should be suppressed. The measured data according to the insert ofFIG. 2 are given for such a contact.

A layer arrangement containing an epitaxial heterostructure contains atleast one interface between an oxidic superconductor and anonsuperconducting oxide (or an oxidic superconductor with a lowcritical temperature). With multiple building up of such a layerarrangement, a multilayer system can be obtained.

The superconductor oxidic material is characterized by a layer structurewhich contains at least oxygen and Cu and has a charge carrier densitybetween 1×10¹⁸ and 5×10²¹ cm⁻³. The nonsuperconductive oxidic layer ischaracterized by a compatibility with the superconductor layer in termsof technology, chemistry and structured and should preferably have acharge carrier density between 1×10¹⁸ and 5×10²¹ cm⁻³.

The modification of the boundary layer in a thin (<10 nm) layer in theregion of the boundary layer can be either structural or also chemical(e.g. doping) or of some other type. It can, for example, also be a verythin (<10 nm) intermediate layer of another material. The result is thata targeted manipulation of the interfacial damage influences theelectron transport characteristics of the abutting materials.

It has been found to be advantageous in general to initially subject thelayer surface to cleaning with the aid of known methods and then tocarry out the modification by treating its surface or by applying afurther modifying layer when the aforementioned procedure is notcompulsory.

What is claimed is:
 1. A superconductive device comprising at least onelayer arrangement consisting of an epitactic first layer of a hightemperature superconductor having at least one CuO₂ plane forming unitcells, an epitactic nonsuperconductive second layer composed ofPrBa₂Cu₃O₇ and different from the material of said first layer, saidfirst and second layers having thicknesses of more than 100 nm, andbonded to said first layer, and a boundary layer formed byion-implantation of one of said first and second layers and chemicallyor physically or both chemically and physically modifying said one ofsaid first and second layers and interposed between said first and saidsecond layers for modifying charge density within said first and secondlayers up to 100 nm therein.
 2. The superconductive device defined inclaim 1 wherein said boundary layer is doped with metal ions.
 3. Thesuperconductive device defined in claim 1 wherein said one of saidlayers is said nonsuperconductive second layer.
 4. The superconductivedevice defined in claim 1 wherein said nonsuperconductive second layerhas a modified surface on a side thereof opposite that along which saidboundary layer is provided and which has been subjected to ionimplantation.
 5. The superconductive device defined in claim 4 whereinsaid surface is bonded to a further high temperature superconductivelayer.
 6. The superconductive device defined in claim 1 wherein aplurality of said layer arrangements are provided in superimposition. 7.The superconductive device defined in claim 1 wherein said device formsa flux transformer.
 8. The superconductive device defined in claim 1wherein said device is a conductive strip crossover.
 9. Thesuperconductive device defined in claim 1 wherein said device forms aJosephson contact.